Multi-cell batteries that are constructed in a broad range of electrochemical systems are typically packaged in cylindrical or prismatic housings. Individual cells are connected in series by conductive links to make the multi-cell batteries. Such construction approaches provide for good sealing of the individual cell compartments and for reliable operation. However, such constructions allocate a large fraction of the multi-cell battery's weight and volume to the packaging and, thus, do not make full use of the potential energy storage capacity of the active components of the cell. For improving battery energy storage capacity on a weight and volume basis, packaging approaches are sought that reduce packaging weight and volume and that provide stable battery performance and low internal resistance.
These objectives have led to the pursuit of a bipolar construction in which an electrically conductive bipolar layer serves as the electrical interconnection between adjacent cells as well as a partition between the cells. In order for the bipolar construction to be successfully utilized, the bipolar layer must be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good electrical contact to the electrodes and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell. These requirements are more difficult to achieve in rechargeable batteries due to the charging potential that can accelerate corrosion of the bipolar layer and in alkaline batteries due to the creep nature of the electrolyte. Achieving the proper combination of these characteristics has proved very difficult.
For maintenance-free operation it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilize flat electrodes and stacked-cell constructions that are structurally poor for containment of the gases present or generated during cell operation. In a sealed cell construction, gases are generated during charging that need to be chemically recombined within the cell for stable operation. To minimize weight of the structures used to provide the gas pressure containment, the battery should operate at relatively low pressure. The pressure-containment requirement creates additional challenges on designing a stable bipolar configuration.
Despite a number of patents and considerable effort at making a bipolar construction for the lead-acid and nickel-cadmium systems such batteries are not commercially available, (U.S. Pat. No. 4,098,967). Construction of a flat metal hydride battery is even more difficult because many of the metal hydride alloys used to make metal hydride batteries operate at elevated hydrogen pressures.
The bipolar construction has been successfully employed in the flat plate construction of the Leclanche Mn0.sub.2 -Zn system as a primary battery, U.S. Pat. No. 4,098,965. Since a primary battery is non-rechargeable, the materials-stability problem is less severe and the aqueous chloride electrolyte may be contained without unreasonable difficulty.
Another problem of prior art electrochemical cells relates to the material problems encountered with metal hydride electrodes. Electrochemically reversible metal hydride electrodes operate by the absorption of hydrogen in the lattice of the metal hydride alloy during electrochemical charging of the cell. A number of alloy formulations have been identified of the so-called AB.sub.5 and AB.sub.2 structure that can function in this manner, for example, as disclosed in U.S. Pat. Nos. 4,488,817 and 4,728,586. To insure reasonable rates of reaction and transport of hydrogen, such electrodes may be prepared from alloy powders typically having an average particle size of about 50 microns. Fabricating an electrode structure from the alloy powders may be accomplished by sintering the metal powders or by using polymeric binders. However, conventional techniques do not yield electrodes that make good and stable contact to the cell face of the conductive outer layer in a bipolar construction. Metal hydride alloys can fragment during repeated cycling as the alloy undergoes expansion and contraction each time the hydrogen enters and leaves the lattice. It is also recognized that oxygen and or the electrolyte can react with the hydride alloy and cause deterioration of the hydrogen storage capacity of the hydride alloy.
The present invention provides a method for achieving the packaging benefits of bipolar construction for rechargeable multi-cell batteries and of overcoming the materials and construction problems of previous approaches. Although the materials of construction for each type of cell are specific to each battery chemistry, the generic bipolar construction disclosed herein may be used for many types of electrochemical cells. In particular, the descriptions and approaches that follow relate specifically to the rechargeable nickel-metal hydride chemistry but may generally be adaptable to other chemistries.
The electrically rechargeable nickel battery electrode that may be used in an electrochemical battery has proven itself to have good cycle life and discharge rate capability in a number of battery systems including nickel/cadmium, nickel/iron, nickel/zinc, nickel/hydrogen and nickel/metal-hydride. The generally accepted reaction at the nickel electrode is as follows: EQU NiOOH+H.sub.2 O+e.sup.- .fwdarw.Ni(OH).sub.2 +OH.sup.-
The nickel active material cycles between the two oxidation states (+2 and +3) during the charge/discharge process. Based on a one-electron change, 3.43 grams of Ni(OH).sub.2 is theoretically reacted per ampere hour of capacity. The excellent cycle life of nickel electrodes can be attributed to the low solubility of the charge/discharge species in the alkaline electrolyte employed and the fact that the active material does not chemically or physically change significantly during cycling. In order for the active material to function in a battery electrode, it must be in electrical contact with an electrode current collector for electron flow and in physical contact with the electrolyte for the electrolytic ion reaction.
The nickel active material is a relatively poor electronic conductor in its two oxidation states, is not cohesive and goes through a modest expansion and contraction between charge and discharge. To overcome these shortcomings practical battery electrodes are fabricated in such a way that the active material is structurally contained to avoid sheathing and held in intimate contact with an electrically conductive component to ensure the flow of electrons from the active material. This has been achieved with pocket electrodes in which a mix of active material and nickel flake or graphite is packed into perforated tubes or pockets, developed by Edison, U.S. Pat. No. 723,450 (1902), and Jungner, Sweden Patent No. 11,132 (1889); sintered electrodes in which active material is chemically or electrochemically impregnated into porous sintered nickel powder, German Patent No. 491,498 (1928); plastic bonded electrodes in which a mix of graphite and active material is bonded together with a Teflon.RTM. binder, U.S. Pat. No. 3,898,099, and pasted electrodes in which nickel hydroxide active material and a binder is pasted into a porous nickel foam or fiber plaque, U.S. Pat. No. 4,844,999.
In the above electrode types, the required combination of properties, specifically including structural containment of the active material, access to electrolyte and electronic contact to the active material is achieved. The relative merits of the different electrode types are evaluated on the basis of capacity per unit volume, capacity per unit weight, cost per unit of capacity, electrode current drain rate capability, cycle life, capacity retention, temperature characteristics, charge efficiency, operating voltage and process hazardous waste.
The characteristics of each electrode type are a function of the weight and volume ratios of active material to additives and support structure, percent utilization of the active material, conductivity of the composite structure and stability of the electrode structure. A given application may place greater emphasis on a particular electrode characteristic since no existing electrode type is superior in all characteristics. Therefore, different electrode types are typically selected for different applications. The limitation of the prior art is that finished electrodes typically contain 30-60% added inert weight, in addition to the weight of the active material and, furthermore, require costly structures and processes. Finished electrodes fall far short of the theoretical energy capabilities of the active material by itself.
The same issues of electrode characteristics, costs and limitation exist with other battery electrode chemistries and the invention described herein is also applicable to other electrode chemistries.